The Goal

Find

The Mistake

Find the mistake:

(Roll the mouse over the math to see a hint in red)

The Correction

(Roll the mouse over the area above to see the correction in blue)

An Explanation

The student has not used the correct formula to find the derivative of inverse tangent, and there's no sign that the chain rule was used at all (we would see a 2x factor somewhere).
The correct derivative formula for the inverse tangent function is: